The present invention relates to sensing schemes for read operations on semiconductor devices, and, more particularly, to a method for programming a reference cell for use in a read operation.
Memory devices, such as random access memory (RAM), read-only memory (ROM), non-volatile memory (NVM) and like, are known in the art. These devices provide an indication of the data which is stored therein by providing an output electrical signal. A device called a sense amplifier is used for detecting the signal and determining the logical content thereof. U.S. Pat. No. 4,916,671 to Ichiguchi describes one such sense amplifier.
In general, prior art sense amplifiers determine the logical value stored in a cell by comparing the output of the cell with a fixed reference voltage level. The aforementioned U.S. Pat. Nos. 6,134,156 and 6,128,226 describe an alternative circuit architecture in which a reference cell is used to generate a reference signal in lieu of a fixed reference voltage value.
When a memory cell is programmed or erased, the signal it generates differs from the reference signal by some margin. Since reading the cell""s state should always result the same (i.e., either programmed or erased depending on the cell""s state), introducing such margin is necessary to overcome imperfections in the reading process and to compensate for drifts in the cell""s threshold voltage (e.g., caused by retention loss or disturbs). A reduction in the original margin due to imperfections in the reading process (e.g., due to operation at different operational conditions) is referred to as xe2x80x9cmargin loss.xe2x80x9d
It is well understood that the placement of a reference signal to which an array cell signal can be compared during sensing can be achieved in a number of ways. When close to ground signals are sensed as in the aforementioned U.S. Pat. Nos. 6,134,156 and 6,128,226 patents, the reference cell signal develops at an intermediate rate between that of a programmed cell and an erased cell. When set this way, the array cells"" signals on one side of the reference signal are determined to be programmed cells, while signals on the other side of the reference signal are determined to be erased cells. For example, array cells generating signals smaller than the reference signal are considered to be programmed and array cells generating signals larger than the reference signal are considered to be erased. Conventionally, such placement is achieved by using a reference cell whose current is between the erased and programmed cells"" current levels. The reference cell""s current level can be controlled by the reference cell""s size, its programming level, or its gate voltage level. Furthermore, if voltage signals are used to detect the cells"" contents, then the reference signal placement can be controlled by providing a different load capacitance on the reference cell compared to that of the array cells. However, if the array and the reference cells differ in their sizes, in their operating gate voltages, or in their loads then some margin loss will be introduced to the sensing scheme. On the other hand, placing the reference cells"" signals by properly programming the reference cells (while operating the array and reference cells at identical conditions) minimizes the sensing scheme sensitivity to operating conditions, process parameters and environmental conditions variations, thereby minimizing the margin loss, if any, that is introduced to the sensing system.
When reference cell placement is by programming, it must be programmed a precise amount in order to achieve its intended purpose. There are difficulties attendant with reliable programming of a reference cell so as to minimize operating margin loss, as well as accurate placement of a programmed reference cell relative to the memory array cells. The present invention provides a method for programming reference cells to minimize margin loss and maximize cycling performance.
The present invention provides a method for programming one or more reference cells, with the programming being performed relative to a prescribed cell on the same die as the reference cell (e.g., a memory cell or a golden bit cell).
According to one aspect of the invention, a method for programming a reference cell for use in an integrated circuit memory having an array of memory cells each exhibiting a native threshold voltage value is described. That method comprising the steps of first locating an address for the memory cell in the array that has the highest native threshold voltage value (VTNH). A reference cell is programmed a predetermined amount and its program state is sensed relative to the VTNH memory cell. The programming and sensing steps are repeated until the sensing step indicates that the reference cell has been programmed an amount sufficient to fail a first preselected read operation.
In a more particular methodology in accordance with this aspect of the invention, the locating step can include the steps of iteratively increasing a gate voltage applied to the memory cells and performing the first preselected read operation at each such applied gate voltage until a final gate voltage is identified at which all the memory cells in the array pass the first preselected read operation. Further, the first preselected read operation can exclude memory cells that have already passed the first preselected read operation at a previously applied gate voltage.
According to another aspect of the invention, a method for programming a reference cell for use in an integrated circuit memory having a plurality of memory cells each exhibiting a native threshold voltage value is described. That method locates an address for the VTNH cell by applying a first gate voltage value at which at least one memory cell fails a first preselected read operation and increasing the applied gate voltage until a final gate voltage value is reached at which each of the memory cells can just pass the first preselected read operation. The reference cell is programmed a predetermined amount and the program state of the reference cell relative to the VTNH memory cell is sensed by performing a second preselected read operation on the reference cell. The programming and sensing steps are repeated until the sensing step indicates that the reference cell has been programmed an amount sufficient to fail the second preselected read operation.
According to still another aspect of the invention, a method for programming a set of reference cells for use in performing respective read operations on an integrated circuit memory having a plurality of memory cells is described. That method locates the VTNH cell, determines a placement for a reference voltage read signal relative to the VTNH cell, places a reference voltage erase verify signal relative to the reference voltage read signal, and places a reference voltage program verify signal relative to the reference voltage read signal.
The foregoing methods can have their sensing steps performed relative to the VTNH memory cell and also relative to a native cell (a golden bit cell) on-board the same die.
The inventive method can be utilized to program a reference cell used with a memory array, a sliced array having one or more columns of memory cells, and redundant or auxiliary arrays.
These and other more specific aspects and features of the present invention can be appreciated from the acompanying Drawing Figures and Detailed Description of Certain Preferred Embodiments.